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What is the real 5NF?

By Millist W. Vincent


this paper is to synthesise some recent research [16-18] which has resolved the question of what is the 'correct' normal form for JDs and to convey to the wider database community the somewhat unexpected results which were uncovered. The first main result is that there is not one 'correct' normal form for JDs but in fact three, with each one corresponding to a different but distinct semantic aim of database design. Surprisingly, we have shown that the necessary and sufficient condition to achieve the semantic aim of eliminating redundancy, the oldest motivation for normalisation, is a new formal form (called key-complete normal form) which has not appeared previously in the literature. The existence of three different normal forms in the case of JDs is in contrast to the case where every JD is a multivalued dependency (MVD). In this latter case, all three normal forms coincide with 4NF [16, 17]. The fact that normal forms for JD is more complex that that for MVDs is consistent with the observation that in general JDs are more subtle and intractable than MVDs. The second main result [16] is that the definition of 5NF which appears in the literature is incorrect because it does not coincide with 4NF in the case of where every JD is an MVD. We show that the difficulty can be overcome if the definition of 5NF is modified to restrict the class of join dependencies that need to be tested in the 5NF condition

Year: 1998
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